Figure 1: Formation of Cystine
Outline
- Disulfide Bridges in Small Proteins
- Intermolecular Disulfide Bridges
Opening Image: Human vascular endothelial growth factor (VEGF) complexed to the extracellular domain of the VEGF receptor. The two identical domains of the receptor are shown in violet.
Introduction
A disulfide bond is a covalent bond between two cysteine residues in proteins. Two cysteine residues which are in close spatial position in the three-dimensional structure of a protein can be oxidized to form a disulfide bond. Disulfide bonds, also called disulfide bridges, are required for the stability and function of a large number of proteins. No denaturant is required to unfold such proteins. Unfolding will occur simply upon the addition of reducing agents, see .
Most proteins that work in the extracellular milieu contain disulfide bonds. Such proteins include circulating hormones, blood-clotting proteins, immunoglobulins, the extracellular domains of cell-surface receptors, to name a few.
Disulfide Bonds in Small Proteins
Endothelin
This 21-amino-acid peptide, discovered in 1989, is the most potent vasoconstrictor yet identified. It is released by vascular endothelial cells. The conformation of this hormone is stabilized by two disulfide bonds between four Cys residues (the CPK color for sulfur is yellow).
Often we are interested only in the number of disulfide linkages in a protein. In viewing these molecules, we often show the model, which draws a tube between Cα and displays the S–S bonds as yellow cylinders between Cα atoms of the cysteine residues.
model. Cross-linking the polypeptide chain with two covalent S–S bonds stabilizes the folded conformation of this important signal molecule.
Disulfide Bonds in Small Proteins
Mating Pheromone
This small, 40-amino-acid protein is secreted by the protozoan Euplotes raikovi.
This small protein is stabilized by three disulfide bonds.
spacefill. Look for holes running through the molecule. Are these cavities filled with water?
Very small proteins which lack a well-packed hydrophobic core are usually stabilized by disulfide bonds.
Intermolecular Disulfide Bonds
Vascular Endothelial Growth Factor
model with disulfide bonds. If you look carefully, you'll find a knot in each subunit.
Intermolecular Disulfide Bonds
Lysozyme
One of the best-known enzymes, lysozyme protects us from bacterial infection. It is a small 14 kDa protein found in tears, nasal mucus, and saliva. It is also present in egg white. Discovered in 1921 by Sir Alexander Fleming, lysozyme attacks the cell walls of certain bacteria, causing the bacteria to burst under their own osmotic pressure. Most of the bacteria killed by lysozyme are not pathogens. However, lysozyme is a primary reason why these bacteria do not become pathogenic.
Lysozyme is an unusually stable protein in large part because it has four S–S bonds. backbone model.
The four disulfide pairs are:
surface model.
The two cysteines in disulfide linkage can be far apart in the primary structure, or may even be located on different chains, as in insulin. The folding of the polypeptide chain can bring the cysteine residues in close proximity and permit covalent linking of the two side chains. A disulfide bond makes a large contribution to the stability of the tertiary structure. For this reason, proteins that are secreted by cells often contain disulfide bonds. The disulfide bonds help stabilize the structure of these proteins in a hostile extracellular environment.
Insulin: Interchain Disulfide Bonds
One of the best-known hormones, insulin is synthesized initially as preproinsulin. The mature insulin is a two-chain molecule with 21 and 30 residues. Thus, insulin consists of only 51 of the initial 110 amino acid residues of preproinsulin.
backbone model. The two chains are connected by two disulfide bonds; in addition, there is an intrachain S–S bridge in the A chain (blue). The B chain shown in purple.
model.
Structure files contain Cartesian coordinates of all the atoms that could be located (modeled) in the electron density map. X-ray crystallographic data also include a temperature factor for each atom in the file. This factor is a measure of the thermal motion of individual atoms in the crystal. Relative values colored red (hot) to blue (cold). In general, side chains have higher temperature factors than backbone atoms, and side chains in loops at the surface exhibit higher temperature factors than those buried in the interior.
The backbone is colored according to temperature colors of the Cα atoms. Disulfide bonds, however, are shown in white.
spacefilled model. Note the absence of really "cold" atoms in this protein. Why do you suppose this is so?